There are several pieces of post-extrusion machinery that are designed to fulfill a specific aspect of extrudate needs. These are very common in the plastic industry, in which the various cutters and take-off systems provide an excellent solution to the specific needs of the extrudate. However, in the food, pet food, and feed industries there is very little developed to meet the needs of the extrudate as it is leaving the die area. These needs include dealing with stickiness, rate of expandability, and high moisture extrudates with needs of special handling and care until the product is dried and solidified. In order to accommodate some of the needs in the food industry, the designs for new types of cutters at the die may vary from use of lasers as cutting devices, to high pressure air cutting which does not subject the product to contact with any materials, to flexible knives and solid, hard, heated knives. Some of these concepts have been tried and are used commonly in plastics, with very few in the food and pet food industries. Some have never found a market or application due to failure of the designs or the weakness of the design in supporting the needs of the product. Variations of material composites of the knives also have been tested, and are used extensively for a given process. The under-oil cutting process of the present invention is based on a process currently in use in the plastics industry, and referred to as underwater cutting.
To accommodate specific design needs of the extrudate, a number of concepts have been developed to support the three dimensional aspects of the extrudate by working with the die design as well as cutting patterns. One idea is the use of short and long knives, one length to partially cut the extrudate while the next set is to cut the extrudate free from the die area. By use of the partial cut, the extrudate may take on a three dimensional phase which can be intriguing to the consumer. Another variation of such a process is to keep the cutter portion consistent and rotate the die area to subject the extrudate to various cutting lengths. Such a cutting technique can result in a rose-like shape with three dimensional petals. Most of the above procedures work the best with products that make use of the vapors under pressure within the extruder and a solid matrix for the bubbles to be formed once the vapor releases the pressure by leaving the die, thus producing expansion in the product while it is being extruded leaving the die.
None of the above processes have focused on the area of the die under which the extrudate is being cut or being introduced to the atmosphere at the moment it leaves the die parameter. At this moment, extrudates, and more specifically the extrudates made up of organic polymers such as cellulose, proteins, sugars, starches and hydrocolloids extruded under high temperatures and pressures (225.degree.-385.degree. F., and 150-1300 psi), are subject to a violent reaction and deformation. On one hand, the gaseous medium within the matrix of the organic polymer is going through a phase change in which the liquid is converted into gas and thus is expanded. At the same time, depending on the temperature of the extrudate, a portion of the gas escapes due to the evaporation from the surface of the extrudate as well as leakage from the inside of the extrudate. The evaporation of the gaseous substance, which in most cases is the water tends, to cool the particulate and, by doing so, reduces the temperature of the particulate. Thus the pellet is expanded and cooled to a point where the temperature of the extrudate no longer provides the necessary energy for the liquid to stay as a gas and the gaseous pressure is not sufficient to overcome the forces of the polymer lining the surface of the bubble. In most instances, organic polymers such as in carbohydrates, starches, proteins or hydrocolloids, where the expansion is taking place due to the release of pressure, there is a certain amount of internal gas exchange with the outside die environment medium, as well as a certain amount of collapse of the expanded extrudate due to cooling and the viscoelastic property of the melted extrudate medium.
The major benefit of the present invention is to meet the needs in the pet food and feed industries for incorporating certain amounts of fat into the precooked extrudate in a manner and to an extent which was not possible by any other means. In the past, methods employed such as spraying oils or fats following extrusion resulted in a maximum of 30% fat content. With today's feed needs to accommodate the requirements of the dairy market which utilizes hormones for higher production of milk, thus necessitating a feed supply containing high protein and high fat for the dairy cattle, as well as market demands for high fat, high carbohydrate feeds for the specialty pet food industry, the present invention has solved these needs by being capable of expanding extrudates up to 90% fat content, if desired.